This invention relates generally to the field of power conversion, and more specifically to a method of power conversion and apparatus which achieves high power factor correction using ripple current mode control.
Electronic equipment and specifically electronic products that rectify and filter the AC line pollute the AC power line with unwanted line current harmonics. These harmonics do not contribute to the transfer of power from the power source to various loads in a power system, which wastes energy in transfer losses due to wire resistance. Unwanted line current harmonics also reduce the useful life of induction motors and transformers, and lead to higher current ratings for protection devices such as fuses or circuit breakers. These higher current ratings lead to costly, larger gauge system wiring.
The unwanted line current harmonics are harmonics that are not present in the AC line voltage. It has been a goal of the power industry to improve the harmonic content of AC line currents and various standards have evolved and continue to evolve to limit the amount of undesirable AC line current harmonics. That is harmonics that are outside of the fundamental harmonic of the AC line voltage.
An ideal load would appear to the AC line as a resistive load. A resistive load results in line currents that match the spectral content and displacement of the AC line voltage. In other words the resistive load results in current waveforms that are identical to the ac line voltage waveform when scaled by a constant equal to 1/R where R is the resistance of the load, and where R is constant or slowly varying with respect to the AC line voltage.
Power factor is a figure of merit for power conversion systems, and is the ratio of the average load power to the maximum possible power that could be delivered to the load for a given Volt-Amp level. The maximum possible power is the product of the RMS value of the AC line voltage and the RMS value of the AC line current. This maximum occurs when the AC load characteristic is purely resistive, resulting in an ideal power factor of one. Passive and active power factor correction methods and devices have been developed to address the need to reduce unwanted line current harmonics.
Passive power factor correction is an approach that uses passive reactive components to counter the effects of reactive components in the load. For example, an inductive motor load produces line currents that are displaced from the line voltage. By adding a compensation capacitor to the load the displacement can be nullified, producing a load that appears resistive to the AC line. However, this approach required large and expensive components, especially for higher power systems.
Active power factor correction typically employs a switch mode power converter such as a boost converter to emulate a resistive load characteristic. In this approach a power inductor is switched between circuit ground and a load filter capacitor in such a way as to present an average AC line current that is proportional to the AC line voltage. The switching rate is many times faster than the frequency of the AC line voltage so that the AC line voltage can be approximated as a constant voltage source over the period of one switching cycle.
Active power factor correction is usually operative in discontinuous or continuous current mode. Discontinuous current mode is simpler to implement because the inductor current returns to zero amperes before the start of each switching interval. This characteristic is advantageous for reducing switching losses, but requires higher peak current to achieve a given average current level producing greater stress on components in the system. Furthermore, discontinuous current mode control inject high frequency line current components on the AC line which require expensive electromagnetic interference (EMI) filters to reduce conducted emissions levels.
In continuous current mode operation the current levels in the inductor can more closely match the AC line voltage harmonics. In continuous current mode there is a ripple, or small variation in the inductor current levels about the desired average current level. The continuous current mode ripple current is much smaller that the ripple seen in the discontinuous current mode systems.
The control methods for power factor correction in continuous current mode require two control feedback loops. One to match the AC line current to a reference current derived from the AC line voltage or inferred from the converter off duration and the output filter voltage, and the other feedback loop to set the power throughput levels based on output voltage feedback. A typical continuous current mode approach requires AC line current sensing for the current loop controller.
Present approaches to power factor correction have many shortcomings. This has lead to research and development of alternate approaches to power factor corrected systems as described previously. However, many problems remain that need to be solved.
Continuous current mode operation is preferred to reduce component electrical stresses, radiated emissions, and achieve higher efficiencies but continuous current mode operation requires continuously monitored AC line current. Current is monitored and compared to a reference waveform in a feedback loop which leads to a trade-off between high signal to noise current sense monitors and achieving a low average power current sensing capability. This trade-off has lead to approaches that are either noise prone or waste power.
Power factor corrected systems operative in discontinuous current mode are possible without the need to monitor the AC source current. However, these systems are limited to discontinuous current mode operation which has high peak current levels, causing unwanted EMI and electrical stresses on components in the system.
In many approaches to power factor correction the AC line voltage is monitored to provide a reference for the current waveform envelop by a high impedance sense line in close proximity to high power electromagnetic fields. The result is that noise is coupled into the control system. Control elements must be placed in close proximity to power elements and shielded adding cost to the approach in order to reduce noise coupling.
As a consequence of the limitations of present approaches to power factor correction, the art continues to seek improvement to power factor correction methods. It would be desirable to provide a method of power factor correction that operates in continuous current mode and does not require an AC current sensing element. It would also be desirable to provide a method of power factor correction that operates in continuous current mode and does not require AC line voltage or current monitoring. It would also be desirable to provide a method of power factor correction that operates in continuous current mode and does not require AC line voltage or AC current monitoring, and permits the controller element to be remotely located far from noise generating high power components.